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The 22 degree offset means that the beam comes off the dish at
an offset angle of 22 deg. With the feed at the lower edge, this means
that the beam comes off the dish at a 22 deg angle upwards. So if the dish
reflector itself is vertical, the beam elevation angle is 22 deg.
For elevation angles lower than 22 deg the dish must be tilted forwards and
downwards. For very low elevation angles the lower edge of the dish
and the back end of the feed support arm may meet up with the mount pole.
In such cases it is necessary to mount the dish upside down with the feed at the
top. This way you can go down to zero elevation angle - which is
applicable if you want to use the dish for terrestrial purposes.
For elevation angles higher than 22 deg you need to tilt the top edge of the
dish backwards. A backward tilt of 68 deg would mean the elevation angle
was 90 deg, with the beam going straight upwards.
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If you want to make yourself an elevation angle scale you
can mark the card in 5 deg increments as follows:
| Angle measured with protractor |
Elevation angle marked |
3
8
13
18
23
28
33
38
41
43
48 |
25
30
35
40
45
50
55
60
63
65
70 |
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The figure shows a "22 deg offset type" dish. The offset
dish is shown set for 63 deg beam elevation angle by tilting the dish
backwards by 41 deg. This 41 deg angle may be set using an
inclinometer or by using a home made inclinometer using a card as below.
A piece of rectangular cardboard has been used as an inclinometer.
You make a pin hole in the top right corner of the card and then insert a
string and small weight (e.g. metal nut) like a plumb bob.
Carefully draw a line parallel to the right edge and through the pin
hole. Then, using a school protractor set out an angle of say:
41 deg and mark this "63
deg elevation" |
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